U.S. patent number 7,294,832 [Application Number 10/537,019] was granted by the patent office on 2007-11-13 for mass separators.
This patent grant is currently assigned to Griffin Analytical Technologies, LLC. Invention is credited to Garth E. Patterson, James Mitchell Wells.
United States Patent |
7,294,832 |
Wells , et al. |
November 13, 2007 |
**Please see images for:
( Certificate of Correction ) ** |
Mass separators
Abstract
In one implementation, processes for designing mass separators
from a series of mass separator electric field data and processes
for designing an ion trap from a range of data pairs and a mass
analyzer scale are provided. Methods for producing mass separators
including ion traps having Z.sub.0/r.sub.0 ratios from about 0.84
to about 1.2 are also provided. Mass spectrometers are al provided
that can include mass separators in tandem with one being an ion
trap having a Z.sub.o/r.sub.o ratio between 0.84 and 1.2. The
present invention also provides methods for analyzing samples using
mass separators having first and second sets of components defining
a volume with a ratio of a distance from the center of the volume
to a surface of the first component to a distance from the center
of the volume to a surface of the second component being between
0.84 and 1.2.
Inventors: |
Wells; James Mitchell
(Lafayette, IN), Patterson; Garth E. (Brookston, IN) |
Assignee: |
Griffin Analytical Technologies,
LLC (West Lafayette, IN)
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Family
ID: |
32469425 |
Appl.
No.: |
10/537,019 |
Filed: |
December 2, 2003 |
PCT
Filed: |
December 02, 2003 |
PCT No.: |
PCT/US03/38587 |
371(c)(1),(2),(4) Date: |
June 01, 2005 |
PCT
Pub. No.: |
WO2004/051225 |
PCT
Pub. Date: |
June 17, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060163468 A1 |
Jul 27, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60430223 |
Dec 2, 2002 |
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Current U.S.
Class: |
250/292;
250/281 |
Current CPC
Class: |
H01J
49/424 (20130101); H01J 49/4255 (20130101) |
Current International
Class: |
H01J
49/42 (20060101) |
Field of
Search: |
;250/292,281 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 336 990 |
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Oct 1989 |
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EP |
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WOX/US03/38587 |
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Sep 2007 |
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WO |
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Other References
Database WPI week 199921; Derwent Publications Ltd., London, GB; AN
1999-250451; XP002416788. cited by other .
Barlow et al., "Determination of analytical potentials from finite
element computations", international Journal of Mass Spectrometry,
vol. 207, Apr. 12, 2001, pp. 19-29. cited by other .
Bollen et al., "ISOLTRAP: a tandem Penning trap system for accurate
on-line mass determination of short-lived istopes", Nuclear
Instruments & Methods in Physics Research, vol. A368, Jan. 11,
1996, pp. 675-697. cited by other.
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Primary Examiner: Nguyen; Kiet T.
Attorney, Agent or Firm: Wells St. John P.S.
Parent Case Text
CLAIM FOR PRIORITY
This application claims priority to U.S. provisional patent
application Ser. No. 60/430,223 filed Dec. 2, 2002, entitled
"Optimized Geometry for Ion Trap."
RELATED PATENT DATA
This application is a 35 U.S.C. .sctn.371 of and claims priority to
PCT International Application Number PCT/US03/38587, which was
filed Dec. 2, 2003 (02.12.03), and was published in English, which
claims priority under 35 U.S.C. .sctn.119 to U.S. Provisional
Patent Application No. 60/430,223 which was filed Dec. 2, 2002
(02.12.02), the entirety of each are incorporated herein by
reference.
Claims
What is claimed is:
1. A mass separator comprising: first and second sets of electrode
components, individual ones of the components comprising a surface,
wherein, in a cross section, the surfaces of the first set of
components oppose each other, the surfaces of the second set of
components oppose each other, and the surfaces of the first and
second sets of components define a volume, the volume comprising a
first distance corresponding to a half a distance intermediate
opposing surfaces of the first of components and a second distance
corresponding to a half a distance intermediate opposing surfaces
of the second set of components, wherein, a ratio of the first
distance to the second distance comprises from about 0.84 to about
1.2; and wherein the mass separator comprises a cylindrical ion
trap and the surface of the first component comprises the surface
of at least one of the end caps of the ion trap and the surface of
the second component comprises the inner surface of the ring
electrode of the ion trap, the cylindrical ion trap comprising an
electrode spacing distance between individual ones of the end caps
and the ring electrode, wherein the electrode spacing distance is
related to the ratio by a spacer maximum factor and the electrode
spacing distance is less than the product of the spacer maximum
factor times the second distance.
2. The mass separator of claim 1 wherein the end caps comprise
stainless steel mesh.
3. The mass separator of claim 1 wherein the first set of
components are orthogonally related to the second set of
components.
4. The mass separator of claim 1 wherein at least one of the end
caps comprises a solid material having a centrally located
aperture.
5. The mass separator of claim 1 wherein at least one of the end
caps comprises mesh.
6. The mass separator of claim 1 wherein at least one of the end
caps further comprises an opening.
7. The mass separator of claim 6 wherein the opening is aligned
with the volume center.
8. The mass separator of claim 1 wherein the mass separator is
coupled to a mass detector.
Description
TECHNICAL FIELD
The present invention relates generally to the field of analytical
detectors and more specifically to mass spectral ion detectors.
BACKGROUND OF THE INVENTION
Mass spectrometry is a widely applicable analytical tool capable of
providing qualitative and quantitative information about the
composition of both inorganic and organic samples. Mass
spectrometry can be used to determine the structures of a wide
variety of complex molecular species. This analytical technique can
also be utilized to determine the structure and composition of
solid surfaces.
As early as 1920, the behavior of ions in magnetic fields was
described for the purposes of determining the isotopic abundances
of elements. In the 1960's, a theory describing fragmentation of
molecular species was developed for the purpose of identifying
structures of complex molecules. In the 1970's, mass spectrometers
and new ionization techniques were introduced which were capable of
providing high-speed analysis of complex mixtures and thereby
enhancing the capacity for structure determination.
It has become desirable to provide mass spectral analysis using
portable or compact instruments. A continuing goal in designing
these instruments is to optimize the components of the
instrumentation.
SUMMARY OF THE INVENTION
According to one embodiment an ion trap is provided comprising a
body having a length and an opening extending from a first end of
the body to a second end of the body, the length having a center
portion; a first end cap adjacent to the first end of the body, the
first end cap having a surface proximate the first end and spaced a
distance from the center portion; a second end cap adjacent to the
second end of the body, the second end cap having a surface
proximate the second end and spaced the distance from the center
portion; and wherein the body and end caps define a volume between
the surfaces of the first and second end caps and within the
opening, the volume comprising the distance and a radius of the
opening, wherein the ratio of the radius to the distance is from
about 0.84 to about 1.2.
An embodiment also provides a mass spectrometer comprising at least
two mass separators in tandem, at least one of the two mass
separators comprising an ion trap having a Z.sub.0/r.sub.0 ratio
between 0.84 and 1.2.
Other embodiments are disclosed as is apparent from the following
discussion.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described below with
reference to the following accompanying drawings.
FIG. 1 is a block diagram of a mass spectrometer according to an
embodiment.
FIG. 2 is a cross-section of a Paul Ion Trap according to an
embodiment.
FIG. 3 is an end view of the cross-section of the Paul ion trap of
FIG. 2 according to an embodiment.
FIG. 4 is a cross-section of a cylindrical ion trap according to an
embodiment.
FIG. 5 is an end view of the cross-section of the cylindrical ion
trap of FIG. 4.
FIG. 6 is a plot of octapole coefficient relative to quadrupole
coefficient as a function of Z.sub.0/r.sub.0 ratio for a CIT having
an electrode spacing of 0.06 cm according to one embodiment.
FIG. 7 is a plot of quadrupole coefficient as a function of
Z.sub.0/r.sub.0 ratio for a CIT having an electrode spacing of 0.06
cm according to one embodiment.
FIG. 8 is a plot of octapole and dodecapole coefficients relative
to quadrupole coefficients as a function of electrode spacing for
five Z.sub.0/r.sub.0 ratios according to one embodiment.
FIG. 9 is a comparison of simulation and experimental mass spectral
data acquired in accordance with one embodiment.
FIG. 10 is simulated mass spectral data acquired using a mass
separator having a Z.sub.0/r.sub.0=0.8.
FIG. 11 is simulated mass spectral data acquired using a mass
separator having a spacing of 2.56 mm.
FIG. 12 is simulated mass spectral data acquired in accordance with
one embodiment.
FIG. 13 is experimental mass spectral data acquired in accordance
with one embodiment.
FIG. 14 is a comparison of the simulated data of FIG. 12 and the
experimental data of FIG. 13 according to an embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
At least some aspects provide processes for designing mass
separators and ion traps, methods for producing mass separators and
ion traps, mass spectrometers, ion traps, and methods for analyzing
samples.
Referring to FIG. 1, a block diagram of a mass spectrometry
instrument 10 is shown. Mass spectrometry instrument 10 includes a
sample preparation ionization section 14 configured to receive a
sample 12 and convey a prepared and/or ionized sample to a mass
analyzer 16. Mass analyzer 16 can be configured to separate ionized
samples for detection by detector 18.
As depicted in FIG. 1, a sample 12 can be introduced into section
14. For purposes of this disclosure, sample 12 represents any
chemical composition including both inorganic and organic
substances in solid, liquid and/or vapor form. Specific examples of
sample 12 suitable for analysis include volatile compounds such as,
toluene or the specific examples include highly-complex
non-volatile protein based structures such as, bradykinin. In
certain aspects, sample 12 can be a mixture containing more than
one substance or in other aspects sample 12 can be a substantially
pure substance. Analysis of sample 12 can be performed according to
exemplary aspects described below.
Sample preparation ionization section 14 can include an inlet
system (not shown) and an ion source (not shown). The inlet system
can introduce an amount of sample 12 into instrument 10. Depending
upon sample 12, the inlet system may be configured to prepare
sample 12 for ionization. Types of inlet systems can include batch
inlets, direct probe inlets, chromatographic inlets, and permeable
or capillary membrane inlets. The inlet system may include means
for preparing sample 12 for analysis in the gas, liquid and/or
solid phase. In some aspects, the inlet system may be combined with
the ion source.
The ion source can be configured to receive sample 12 and convert
components of sample 12 into analyte ions. This conversion can
include the bombardment of components of sample 12 with electrons,
ions, molecules, and/or photons. This conversion can also be
performed by thermal or electrical energy.
The ion source may utilize, for example, electron ionization (EI,
typically suitable for the gas phase ionization), photo ionization
(PI), chemical ionization, collisionally activated disassociation
and/or electrospray ionization (ESI). For example in PI, the photo
energy can be varied to vary the internal energy of the sample.
Also, when utilizing ESI, the sample can be energized under
atmospheric pressure and potentials applied when transporting ions
from atmospheric pressure into the vacuum of the mass spectrometer
can be varied to cause varying degrees of dissociation.
Analytes can proceed to mass analyzer 16. Mass analyzer 16 can
include an ion transport gate (not shown), and a mass separator 17.
The ion transport gate can contain a means for gating the analyte
beam generated by the ion source.
Mass separator 17 can include magnetic sectors, electrostatic
sectors, and/or quadrupole filter sectors. More particularly, mass
separators can include one or more of triple quadrupoles,
quadrupole ion traps (Paul), cylindrical ion traps, linear ion
traps, rectilinear ion traps (e.g., ion cyclotron resonance,
quadrupole ion trap/time-of-flight mass spectrometers), or other
structures.
Mass separator 17 can include tandem mass separators. In one
implementation at least one of two tandem mass separators can be an
ion trap. Tandem mass separators can be placed in series or
parallel. In an exemplary implementation, tandem mass separators
can receive ions from the same ion source. In an exemplary aspect
the tandem mass separators may have the same or different geometric
parameters. The tandem mass separators may also receive analyte
ions from the same or multiple ion sources.
Analytes may proceed to detector 18. Exemplary detectors include
electron multipliers, Faraday cup collectors, photographic and
stimulation-type detectors. The progression from analysis from
inlet system 3 to detector 7 can be controlled and monitored by a
processing and control unit 20.
Acquisition and generation of data according to the present
invention can be facilitated with processing and control unit 20.
Processing and control unit 20 can be a computer or mini-computer
that is capable of controlling the various elements of instrument
10. This control includes the specific application of RF and DC
voltages as described above and may further include determining,
storing and ultimately displaying mass spectra, Processing and
control unit 20 can contain data acquisition and searching
software. In one aspect such data acquisition and searching
software can be configured to perform data acquisition and
searching that includes the programmed acquisition of the total
analyte count described above. In another aspect, data acquisition
and searching parameters can include methods for correlating the
amount of analytes generated to predetermined programs for
acquiring data.
Exemplary ion traps are shown in FIG. 2-5. Referring to FIG. 2, a
Paul ion trap 30 is shown that includes a ring electrode 32
situated between two end-cap electrodes 34. Trap 30 can have a
toroidal configuration. As shown in FIG. 3, a cross section of Paul
ion trap 30 (e.g., hyperbolic cross-section) shows ring electrode
32 and end caps 34. In this cross-section, ring electrode 32 can be
characterized as a set of components and end caps 34 can be
characterized as a set of components. Ring electrode 32 includes an
inner surface 36 and end caps 34 include an inner surface 38. Ring
electrode 32 and end caps 34 define a volume 40 having a center 42.
Inner surface 36 is spaced a distance 46 corresponding to half a
distance intermediate opposing surfaces 36. Distance 46 can be
referred to as r.sub.0. Inner surface 38 is spaced a distance 48
half a distance intermediate opposing surfaces 38. Distance 48 can
be referred to as Z.sub.0.
Referring to FIG. 4, a cylindrical ion trap (CIT) 50 is shown. CIT
50 can include a ring electrode 52 having an opening 53.
Configurations of ring electrode 52 other than the exemplary
depicted ring structure are possible. For example, ring electrode
52 can be formed as an opening a body of material having any
exterior formation. Ring electrode 52 can be situated between two
end-cap electrodes 54. In an exemplary implementation, electrode 52
can be centrally aligned between electrodes 54.
In one implementation, electrodes 54 can be aligned over and
opposing opening 53. Electrodes 54 can be flat and made of a solid
material having an aperture 56 therein. Stainless steel is an
exemplary solid material while other materials including
non-conductive materials are contemplated. Aperture 56 may be
centrally located. Electrodes 54 can include multiple apertures 56.
Individual electrodes 54 may also be constructed either partially
or wholly of a mesh. An exemplary cross-section of CIT 50 is shown
in FIG. 5.
Referring to FIG. 5, ring electrode 52 includes an inner surface
58. Surface 58 can be substantially flat or uniform. End caps 54
have an inner surface 60. Surface 60 can be substantially flat or
planar. In this cross-section ring electrode 52 can be
characterized as a set of components and end caps 54 can be
characterized as a set of components, each having surfaces 58 and
60 respectively. In an implementation, surfaces 58 oppose each
other and surfaces 60 oppose each other. Surfaces 58 and surfaces
60 can also be orthogonally related. Ring electrode 52 and end caps
54 define a volume 62 which may have a center 64. In one
implementation, openings 56 of end caps 54 can be aligned with
center 64. Inner surface 58 is spaced a distance 68 corresponding
to half a distance intermediate opposing surfaces 58. Distance 68
can be referred to as r.sub.0 and the radius of opening 53. Inner
surface 60 is spaced a distance 70 corresponding to half a distance
intermediate opposing surfaces 60. Distance 70 can be referred to
as Z.sub.0. Electrode 52 further includes a half height 72. CIT 50
can have electrode spacing 74 between an end surface 76 of
electrode 52 and surface 60. Spacing 74 can be the difference
between distance 70 and half height 72. In one implementation, half
height 72 can be considered twice the length of electrode 52 with
the center of the length being aligned with center 64.
Aspects are described below with respect of the embodiment of FIG.
5 although it is to be understood that the below discussion is also
applicable to the embodiment of FIG. 3 or other constructions.
Generally, analytes can be stored or trapped using mass separator
17 such as an ion trap through the appropriate application of
radio-frequency (RF) and direct current (DC) voltages to the
electrodes. For example, with respect to the embodiment of FIG. 5,
and by way of example only RF voltage can be applied to ring
electrode 52 with end cap electrodes 54 grounded. Ions created
inside volume 62 or introduced into volume 62 from an sample
preparation ionization section 14, for example, can be stored or
trapped in an oscillating potential well created in volume 62 by
application of the RF voltage.
In addition to storage, analytes can be separated using mass
separator 17 such as an ion trap. For example, and by way of
example only, RF and DC voltages can be applied to electrodes 52,
and 54 in such a way to create an electric field in volume 62 that
trap a single (m/z) value analyte at a time. Voltages can then be
stepped to the next m/z value, changing the electric field in
volume 62, wherein analytes having that value are trapped and
analytes having the previous value are ejected to a detector. This
analysis can continue step-wise to record a full mass spectrum over
a desired m/z range.
According to an exemplary aspect, the RF and DC voltages can be
applied to electrodes 52, 54 in such a way to create electric
fields in volume 62 trapping a range of m/z valued analytes
simultaneously. The voltages are then changed so that the trapped
analytes eject from the ion trap to an external detector in an m/z
dependent manner. For example, where no DC is applied and the RF
amplitude is increased in a linear fashion, ions of increasing m/z
can eject from the trap to a detector. Supplementary voltages may
be applied during the RF amplitude ramp (or during scans of other
parameters such as RF frequency) to influence ion ejection to the
detector. For example, an alternating current (AC) voltage may be
applied at the appropriate frequency to resonantly excite the ions
and cause their ejection in a process referred to as resonance
ejection.
According to another implementation, the RF and DC voltages can be
applied to electrodes 52, 54 in such a way that a range of m/z
values are trapped simultaneously or only a single m/z value is
trapped. The ions are detected by their influence on some form of
receiver circuit as they undergo characteristic motion in volume
62. Exemplary receiver circuits include circuits that can receive
an image current induced by a charged ion cloud on electrodes 52
and/or 54 or on a supplementary electrode and can measure the image
current related to the m/z values of the ions.
Exemplary mass separators can be designed to provide optimum mass
analysis performance including performance in the mass-selective
instability and resonance ejection modes of operation. According to
exemplary implementations, an electric field of volume 62 can be
controlled by manipulation of mass separator geometry to increase
performance. The mass separator geometry can include parameters
such as Z.sub.0, r.sub.0, half height, and/or electrode spacing.
The electric field can include a quadrupole field, higher order
electric fields or other fields. In exemplary implementations the
quadrupole field and higher order fields can be present in volume
62 and may influence analyte motion in volume 62 before and during
mass analysis.
According to some embodiments, mass separator geometry parameters
are selected to provide increased or optimum performance with
respect to a mass spectrometer. The discussion proceeds with
respect to an initial method of providing mass separator electric
field data. The mass separator electric field data includes data
sets of mass separator geometric parameters and corresponding
expansion coefficients. According to one implementation a list of
mass separator geometric parameters can be generated (e.g.,
Z.sub.0, r.sub.0) and applied to Equations 1, 2, and/or 3 below to
generate the corresponding expansion coefficients thereby creating
the data sets. In one aspect, a designer may select possible values
of the geometric parameters for application to the equation for
determining corresponding coefficients. Other methods of generating
the values of the geometric parameters are possible. According to
an exemplary aspect the list is applied to equation 3 below.
An exemplary expression for the potential in an exemplary
cylindrical ion trap with no spacing 74 between ring end surface 76
and end-cap electrodes surface 60 and grounding the end cap
electrodes 54 with RF voltage applied to ring electrode 52 was
developed by Hartung and Avedisian and is given in Equation 1:
.PHI..function..times..infin..times..times..times..times..times..function-
..times..times..times..times..times..times..function..times..times..times.
##EQU00001##
In this expression, J.sub.0 and J.sub.1 are Bessel functions of the
first kind, and x.sub.jr.sub.0 is the j.sup.th zero of J.sub.0(x).
In one implementation, Equation 1 may be expanded in spherical
harmonics to yield Equation 2.
.PHI..times..times..PHI..times..function..times..function..times..times..-
function..times..times..times..times..times..times.
##EQU00002##
In an exemplary implementation, Equation 2 shows that the electric
field in the described CIT may be considered as a superposition of
electric fields of various order, or pole ("multipole expansion").
The expansion coefficients for A.sub.n where n=0 4 in Equation 2
correspond to the monopole, dipole, quadrupole, hexapole, and
octapole components respectively, and the relative magnitude of the
coefficients can determine the relative contribution of each field
to the overall electric field in the described CIT. According to
one implementation, when only the coefficients for n=0 and n=2 are
nonzero, the electric field can be considered purely quadrupolar.
The even ordered coefficients can be calculated from Equation 3 of
Kornienko et al.
.times..times..function..times..times..infin..times..times..times..times.-
.times..times..times..function..times..delta..times..times..times.
##EQU00003##
Here, .delta..sub.n,0 is unity if n=0 and is otherwise zero.
According to another method of providing the mass separator
electric field data, the corresponding expansion coefficients can
be generated numerically from a list of provided geometric
parameters using a Poisson/Superfish code maintained at Los Alamos
National Laboratory (The Poisson/Superfish code is available at
http://laacg1.lanl.gov/laacg/services/possup.html; see also,
Billen, J. H. and L. M. Young. Poisson/Superfish of PC Compatibles,
in Proceedings of the 1993 Particle Accelerator Conference, 1993,
Vol. 2 page 790 792; incorporated herein by reference) coupled with
a CalcQuad/Multifit program available in the academic lab of
Professor R. Graham Cooks, Purdue University, West Lafayette, Ind.
In an exemplary implementation the geometric parameters (e.g.,
Z.sub.0, r.sub.0) as well as a potential applied to each component
can be entered into a program utilizing the Poisson/Superfish code.
The Poisson program can cover volume 62 within the specified
geometric parameters with a mesh and then calculate a potential at
each point on the mesh corresponding to the specific geometric
parameters and corresponding potentials applied to each component
(e.g., Poisson electric field data). Harmonic analysis of the
Poisson electric field data can then be carried out by inputting
the Poisson electric field data into the CalcQuad/Multifit program
to yield the expansion coefficients for each of the geometric
parameters.
Exemplary data sets can include all of the coefficients (e.g., n=0
8) described above as well as the corresponding geometric
parameters (e.g., Z.sub.0/r.sub.0). In certain aspects the data
sets can include octapole and dodecapole expansion
coefficients.
In one embodiment, a range of geometric parameters are selected
from the data set that correspond to positive octapole coefficients
and the least negative docecapole coefficients. For example, and by
way of example only, higher-order fields give large contributions
to the overall field resulting in significant degradation of the
performance of the mass separator in the mass selective instability
mode, particularly if the higher order coefficients are opposite in
sign from the A.sub.2 term. In one implementation this can be
balanced by a small octapole superposition
(A.sub.8/A.sub.2.ltoreq.0.05), which has the same sign as the
A.sub.2 term (i.e., positive as shown in Equation 2), which may
improve performance by off-setting effects of electric field
penetration into end-cap apertures 56 that may be present to allow
for entrance and egress of ions and/or ionizing agents such as
electrons. Exemplary data pairs having this positive octapole
coefficient, typically have a negative dodecapole (e.g.,
.gtoreq.-0.18, from 0 to -0.2, or .gtoreq.-0.05) coefficient. Data
sets having large negative dodecapole coefficients can have
corresponding mass separator geometries that subtract from the
overall electric field and hence degrade trapping efficiency and
mass separator performance. In an exemplary implementation,
minimizing the dodecapole coefficient while providing adequate
octapole coefficient can off-set the effect of the negative
dodecapole superposition to some extent. In another exemplary
implementation, a larger percentage of positive octapole can
optimize CIT 50 performance. The exemplary use of the positive
octapole coefficient and the least negative dodecapole coefficient
can provide an initial range of ratios.
The range of ratios may be further refined in one example by
identifying a minimum and a maximum of the ratios for a given value
of spacing 74. Referring to FIG. 6, a plot of octapole relative to
quadrupole coefficients (A.sub.4/A.sub.2) as a function of
Z.sub.0/r.sub.0 using an exemplary spacing parameter of 0.06 cm
illustrates that the Z.sub.0/r.sub.0 ratio should be greater than
0.84 to give positive octapole with a spacing of 0.06 cm between
the electrodes. Referring to FIG. 7, quadrupole (A.sub.2) as a
function of Z.sub.0/r.sub.0 at an exemplary 0.06 cm spacing
illustrates that as the Z.sub.0/r.sub.0 ratio increases, the
quadrupole field weakens requiring higher RF amplitude to achieve
the same m/z analysis range. At Z.sub.0/r.sub.0.about.1.2, roughly
twice the voltage would be needed to perform mass analysis over a
given range than would be needed in an ideal trap (A.sub.2=1).
Accordingly, in one embodiment a minimum Z.sub.0/r.sub.0 ratio of
0.84 and a maximum of 1.2 are defined and may be used in geometries
having spacing 74 other than 0.06 cm.
At least one aspect also defines another geometric parameter in
terms of spacing 74 intermediate the electrodes. For example, an
increase in the space between electrodes (decrease of half-height)
can be used to optimize the field by minimizing the negative
dodecapole coefficient. FIG. 8 demonstrates A.sub.n/A.sub.2 as a
function of various Z.sub.0/r.sub.0 ratios. As illustrated in FIG.
8, for each value of Z.sub.0/r.sub.0, as the spacing is increased,
a value of spacing 74 (also referred to as spacer value) is reached
where the octapole coefficient A.sub.4 crosses zero and becomes
negative. These spacer values at the zero crossings give a maximum
value of spacing 74 that can be used for a given Z.sub.0/r.sub.0.
These spacer maximum values and corresponding Z.sub.0/r.sub.0
values in the range defined above correspond to the respective
zero-crossings in FIG. 8. Above a Z.sub.0/r.sub.0 ratio of 1, the
relationship between Z.sub.0/r.sub.0 and the spacer maximum values
may be essentially linear, with the spacer maximum values equal to
1.2(Z.sub.0/r.sub.0)-0.77 cm.
An exemplary range of data pairs comprising Z.sub.0/r.sub.0 ratios
and spacer maximum factors is shown in Table 1 below. The spacer
maximum factors of the data pairs are usable to calculate spacer
maximum values for respective Z.sub.0/r.sub.0 ratios to ensure
positive octapole superposition. In one embodiment, the spacer
maximum factors are scaled to yield the spacer maximum values. For
example, a spacer maximum factor may be multiplied by a scaling
factor (e.g., r.sub.0) to define the spacer maximum value for a
respective ratio. The scaling factor can include scales the .eta.m,
.mu.m, mm, or cm, for example. In the described example the spacer
maximum factor is multiplied by r.sub.0 to achieve scaling and
determine the resultant spacer maximum value.
TABLE-US-00001 TABLE 1 Z.sub.0/r.sub.0 Spacer Maximum Factors 0.84
0.08 0.86 0.16 0.88 0.22 0.90 0.26 0.92 0.30 0.94 0.33 0.96 0.36
0.98 0.39 1.00 0.42 1.02 0.45 1.04 0.47 1.06 0.50 1.08 0.52 1.10
0.55 1.12 0.57 1.14 0.59 1.16 0.62 1.18 0.64 1.20 0.66
According to an embodiment, a mass separator may be produced by
aligning the first and second sets of components as shown and
described in FIG. 5 above with a ratio of Z.sub.0 to r.sub.0 of
from about 0.84 to about 1.2. In one example, a desired r.sub.0 and
Z.sub.0/r.sub.0 ratio may be chosen based upon design criteria
(e.g., available RF power supply, gas-tightness, gas throughput,
minimization of gas pumping). Z.sub.0 is determined from the
selected r.sub.0 and ratio. The spacing 74 is determined from the
maximum spacer factor times the scaling factor (e.g., r.sub.0). The
utilized spacing 74 may be equal to or less than the maximum spacer
factor times r.sub.0 in one embodiment.
Instrument 10 can be calibrated with a known composition such as
perfluorotri-n-butylamine (pftba) or perfluorokerosene. Once
calibrated, the instrument can provide mass spectra of analytes
produced according to the methods described above.
Simulation of instruments 10 designed in accordance with disclosed
aspects versus other designs is provided below. The results of the
simulations are provided in FIGS. 9 12 and 14.
Mass spectral data simulations were performed using an ITSIM 5.1
program available from the laboratory of Prof. R. Graham Cooks at
Purdue University. (Bui, H. A.; Cooks, R. G. Windows Version of the
Ton Trap Simulation Program ITSIM: A Powerful Heuristic and
Predictive Tool In Ion Trap Mass Spectrometry J. Mass Spectrom.
1998, 33, 297 304, herein incorporated by reference). The ITSIM
program allows for the calculation of trajectories (motion paths)
of ions stored in ion trap mass spectrometers, including
cylindrical ion traps (CITs). The motion of many thousands of ions
can be simulated, to allow for a statistically valid, realistic
comparison of the simulated ion behavior with the data that are
obtained experimentally. Full control of experimental variables,
including the frequency and amplitude of the RF trapping voltage
and the frequencies and amplitudes of additional waveforms applied
to the ion trap end caps is provided by the simulation program. A
collisional model that allows for simulation of the effects of
background neutral molecules present in the ion trap that may
collide with the ions is also provided. To perform a simulation,
the following steps may be performed: 1) the characteristics (e.g.
mass, charge, etc.) of the ions to be simulated are specified, 2)
the characteristics of the ion trap (e.g. size) are specified, 3)
the characteristics of the experiment to be simulated (e.g.
voltages applied to the CIT) are specified, and 4) the motion of
the ions under these conditions are calculated using numerical
integration. In the sections that follow, exemplary details for
each of these steps is given.
1) The Ions
Three ensembles of ions were created to simulate the ions generated
via electron ionization of toluene (C.sub.7H.sub.8). The ions were
generated randomly in time during the first three microseconds of
the simulation, with the characteristics detailed in Table 2:
TABLE-US-00002 TABLE 2 Characteristics of ions in simulation data
Ion Ensemble 1 Ion Ensemble 2 Ion Ensemble 3 mass 65 Da 91 Da 92 Da
(m) Charge 1 1 1 (z) Number 250 1500 750 of ions initial 0 .+-. 0.3
mm, 0 .+-. 0.3 mm, 0 .+-. 0.3 mm, radial initial 0 .+-. 0.15 mm, 0
.+-. 0.15 mm, 0 .+-. 0.15 mm, axial initial 0 m/sec. 0 m/sec. 0
m/sec. veloc- ity
2) The Cylindrical Ion Traps
To yield the most accurate comparison between the simulation and
the experiment, the cylindrical ion traps used in the simulations
described here were defined by calculating an array of potential
values for the specific CIT geometry under study. This method
allows for the effects of each geometry detail, such as electrode
spacing and end-cap hole size, to be most accurately represented.
To achieve this using the ITSIM program, the geometric coordinates
for each electrode of the trap are specified as x,y pairs in a text
file, together with the potential applied to each electrode. This
file can then be loaded into a CreatePot program (available from
the laboratory of Professor R. Graham Cooks, Purdue University,
West Lafayette, Ind., and based on the Poisson/Superfish code
described above) that calculates the potential at each point on a
rectangular grid within the ion trap volume, and this array of
potential points is then loaded into memory for use in the ion
trajectory calculation. For the simulations described here, a grid
of approximately 100,000 points was used to represent the potential
distribution in the CIT. Before the start of a simulation, the
components of the electric field vector are obtained by taking the
derivative of the potentials on the grid points using centered
differencing. During the simulation, the electric field is
determined at each time step for each ion position by bilinear
interpolation from the electric field components on the adjacent
grid points.
For the simulation data shown below, each aspect of the CIT
geometry was kept constant except for the parameter under test.
Potential array files were generated for each geometry and used to
simulate the trajectories of the same ensembles of ions, as defined
above, using the same simulation conditions defined below. In this
way, the effects of the geometry change on the ion motion, and
ultimately on the mass spectrum, could be measured.
3) The Characteristics of the Experiment Simulated
An ion trap experiment is defined by the voltages applied to the
electrodes of the trap, and how those voltages vary as a function
of time. For the simulations performed here, the voltages were
applied in two segments, with a total simulation length of 5.13 ms.
The details of the voltages applied during each segment are given
in Table 3.
TABLE-US-00003 TABLE 3 Segment 1 (0.5 ms Segment 2 (4.63 ms
Electrode duration) duration) Ring Sine Sine Freq: 1.5 MHz Freq:
1.5 MHz Amp: constant to yield trap Amp: ramped from low-mass
cutoff (LMCO) = LMCO 50 to LMCO 100 50 (actual voltage amplitude
(actual voltage varied with geometry such varied, scan rate that
lowest mass trapped at was always 10.8 Da/ms) q.sub.z = 0.64 was
always m/z 50) End Caps no voltage applied Sine Freq: 375 kHz Amp:
ramped from 1.84 V to 3.41 V (chosen to match experiment)
Segment 1 is a 0.5 ms stabilization time, to allow the ions to come
to equilibrium with the background gas through collisions. Segment
2 is a mass analysis ramp using the mass selective instability mode
with resonance ejection. The trapping voltage on the ring electrode
is ramped in amplitude during this segment to bring ions to
resonance with the voltage applied to the end caps, in order of m/z
ratio. When the ions reach the resonance point, they are excited by
the voltage on the end caps and are ejected from the trap.
The simulations performed here included the effects of background
gas present in the ion trap. The gas was assumed to be mass 28
(e.g. nitrogen to simulate an air background) at a temperature of
300 K and a pressure of 6.times.10.sup.-5 Torr, to match the
experiments. At each time step of the simulation, a buffer gas atom
is assigned a random velocity generated from a Maxwell-Boltzmann
distribution. A random number from a uniform distribution is then
compared to the collision probability to determine if a collision
occurs. The collision probability is calculated assuming a Langevin
collision cross section, with the hard-sphere radius of the ions
equal to 50 .ANG..sup.2 and the polarizability of the neutral gas
equal to 0.205 .ANG..sup.3. The simulation assumes that the gas
velocity is randomly distributed, and also assumes that any
scattering of the ion trajectories that may occur is in a random
direction. Only elastic collisions are considered, i.e. only
kinetic energy, but not internal energy, is transferred during the
collision.
4) Calculation of Ion Motion
ITSIM calculates the trajectories of each ion in the ensemble by
numerically integrating the equation of motion under the conditions
specified above. When an ion leaves the ion trap volume, or at the
end of the simulation, the location of each ion, and the time it
has left the trap if applicable, is recorded. For the simulations
performed here, the integration was performed using a fourth-order
Runge-Kutta algorithm with a base time step size of 10 ns. The
voltages applied to the traps were varied as described above, and
the location of each ion in the trap was calculated every 10 ns.
For the simulations performed here, most of the ions had ejected
from the trap through the end-cap holes, and hence were recorded to
have left the trap and struck a "detector" placed just outside the
trapping volume.
In the mass-selective instability with resonance ejection mode of
operation which is simulated here, ions are ejected from the ion
trap in order from lowest to highest m/z ratio, as described above.
By plotting the ejection time of the ions as a function of ion
number, a mass spectrum of the ions can be generated. The simulated
data for ion number at the detector vs. ejection time were exported
to Excel for plotting and calibration to generate the mass spectra
given in the figures below.
Experimental data was also obtained from exemplary instruments 10
fabricated according to aspects of the disclosure. Experimental
results are shown in FIGS. 9, 13, and 14.
Experimental Details
The experimental data given in the figures below was generated on a
Griffin Analytical Technologies, Inc. Minotaur Model 2001A CIT mass
spectrometer. (Griffin Analytical Technologies, West Lafayette,
Ind. (Griffin)). The CIT used in the Griffin mass spectrometer to
record the data presented below has a ring electrode radius,
r.sub.0 of 4.0 mm, a center-to-end cap spacing, Z.sub.0 of 4.6 mm,
and a ring-to-end cap spacing of 1.28 mm. The CIT, along with the
electron generating filament and the lenses used to transport the
electrons to the CIT for ionization, are housed in a vacuum chamber
that is pumped by a Varian V7OLP turbomolecular pump, backed by a
KNF Neuberger 813.5 diaphragm pump. The pressure inside this
chamber can be set using a Granville-Phillips Model 203 variable
leak valve; for the data collected here, the chamber pressure was
set to 6.times.10.sup.-5 Torr of ambient room air, as measured on a
Granville-Phillips 354 Micro-Ion.RTM. vacuum gauge module.
With this instrument, volatile gas-phase samples are introduced
into the vacuum chamber via a polydimethylsiloxane (PDMS) capillary
membrane located inside the chamber. Organic compounds, such as
toluene, are drawn through the inside of the membrane, permeate
into the membrane material, and then desorb from the outside
surface of the membrane into the vacuum chamber. The main
constituents of air, such as oxygen and nitrogen, are rejected by
the membrane and hence do not enter the vacuum chamber. The analyte
molecules that enter the vacuum chamber are ionized inside the CIT
by an electron beam that is generated from a heated filament and is
then directed into the trap with a set of three lenses. The trapped
ions are allowed to cool via collisions with background air, and
are then scanned from the trap to an external detector in the
mass-selective instability with resonance ejection mode as
described above.
Toluene was introduced to the instrument by drawing the headspace
vapors of the neat liquid through a one centimeter PDMS membrane at
a flow rate of approximately 2 L/min using a KNF Neuberger MPU937
diaphragm pump. The membrane was at ambient temperature. The
toluene molecules were ionized in the CIT for 50 ms with the 1.5
MHz trapping RF set to a voltage that corresponded to a LMCO in the
trap of m/z 50 (note that for the Griffin CIT, the LMCO values are
specified for q.sub.z=0.64, not q.sub.z=0.908 as is typical for
most standard ion traps). The ions were then allowed to cool for 25
ms at LMCO 50 before mass analysis. For mass analysis, the RF on
the ring electrode was ramped from a LMCO of 50 to a LMCO of 150,
at a scan rate of 10.7 Da/ms. During mass analysis, the end cap
sine voltage of 375 kHz was ramped in amplitude from a starting
value of 0.95 V to 1.85 V. Note that the end caps are connected in
such a way that when one end cap has a positive voltage applied,
the other has a corresponding negative voltage applied, so that the
potential between the end caps is actually twice the amplitude of
the voltage applied between each end cap and ground. This accounts
for the factor-of-two difference in the end cap voltage specified
here in the experimental section and that specified above in the
simulations. The ions were detected with a combination conversion
dynode/electron multiplier detector. The dynode was held at -4 kV,
and the electron multiplier at -1.2 kV.
Simulation and Experimental Data
FIG. 9 is a comparison of simulated and experimental mass spectra
for perfluoro tributalamine (PFTBA) collected under identical
conditions using a cylindrical ion trap with Z.sub.0=4.6 mm,
r.sub.0=4.0 mm (Z.sub.0/r.sub.0=1.15), and electrode spacing=1.28
mm.
FIG. 10 is a simulated mass spectrum of toluene calculated for a
cylindrical ion trap with Z.sub.0=3.2 mm, r.sub.0=4.0 mm
(Z.sub.0/r.sub.0=0.8), and spacing=0.6 mm, illustrating that when
the condition 0.84 is not met, the mass spectral performance of the
CIT is poor; i.e. the peaks are broadened and are not
well-resolved.
FIG. 11 is a simulated mass spectrum of toluene calculated for a
cylindrical ion trap with Z.sub.0=4.6 mm, r.sub.0=4.0 mm
(Z.sub.0/r.sub.0=1.15), and spacing=2.56 mm, illustrating that when
the spacer is greater than that defined in Table 1 for this value
of Z.sub.0/r.sub.0 the mass spectral performance is poor; i.e. the
peaks are broadened and are not well-resolved.
FIG. 12 is a simulated mass spectrum of toluene calculated for a
cylindrical ion trap with Z.sub.0=4.6 mm, r.sub.0=4.0 mm
(Z.sub.0/r.sub.0=1.15), and spacing=1.28 mm, illustrating that when
the spacer is within the range defined in Table 1 for this value of
Z.sub.0/r.sub.0, the mass spectral performance is improved; i.e.
the peaks are narrower and more defined, and the signals for ions
of m/z 91 and m/z 92 are well-resolved.
FIG. 13 in an experimental mass spectrum of toluene obtained on the
Griffin mass spectrometer using a cylindrical ion trap with
Z.sub.0=4.6 mm, r.sub.0=4.0 mm (Z.sub.0/r.sub.0=1.15), and
spacing=1.28 mm, illustrating that, when the CIT is constructed
according to the geometry specifications defined above, the mass
spectral performance is improved.
FIG. 14 is a comparison of the simulated and experimental data from
FIGS. 12 and 13.
The invention has been described in language more or less specific
as to structural and methodical features. It is to be understood,
however, that the invention is not limited to the specific features
shown and described, since the means herein disclosed comprise
preferred forms of putting the invention into effect. The invention
is, therefore, claimed in any of its forms or modifications within
the proper scope of the appended claims appropriately interpreted
in accordance with equitable doctrines.
In compliance with the statute, the invention has been described in
language more or less specific as to structural and methodical
features. It is to be understood, however, that the invention is
not limited to the specific features shown and described, since the
means herein disclosed comprise preferred forms of putting the
invention into effect. The invention is, therefore, claimed in any
of its forms or modifications within the proper scope of the
appended claims appropriately interpreted in accordance with the
doctrine of equivalents.
* * * * *
References